This invention relates generally to spectrometers and spectral sensors. More particularly, it relates to novel types of compact spectrometers based on the Talbot effect.
Most existing spectrometers are bulky because of their use of gratings, prisms or large interferometers. These spectrometers are not suitable for a variety of potential applications which require very compact spectrometers, e.g., spectrometer arrays. Compact spectrometer arrays would also benefit from spectrometers having a linear architecture, so that each pixel of an image may be analyzed by its own spectrometer. In order to be useful, such spectrometers would need to provide adequate resolution and be sufficiently inexpensive to manufacture. A technique for wavelength measurement has been described by A. W. Lohmann, in Proceedings of the Conference on Optical Instruments and Techniques, London (Wiley, N.Y., 1961), p. 58. A spectrometer built according to Lohmann""s teaching, however, would not be sufficiently compact for many applications and would have moving parts that would decrease its reliability. There is therefore a need for spectrometers that are reliable, compact, have a linear architecture, have high resolution, and are inexpensive to manufacture.
The present invention provides compact, inexpensive, nanometer-resolution spectrometers that are designed to sense, monitor, and process the spectral content of images. These spectrometers are ideally suited for hand-held spectral imaging and sensing systems. The advance of MEMS (Micro-Electro-Mechanical-Systems) technology has made possible various types of small spectrometers, including Fabry-Perot interferometers, grating-based spectrometers, and standing wave spectrometers. This invention, however, provides a novel type of miniaturized spectrometer based on the Talbot effect.
According to one aspect of the invention, the spectrometers realize a novel method for determining the spectrum of light based on the Talbot effect. Light to be analyzed is passed through a spatially periodic object, thereby generating a series of Talbot images. The intensities of these Talbot images at different optical distances from the spatially periodic object are then detected, and Fourier transformed to determine the spectrum of the light. In one embodiment, the detector comprises a spatial masking pattern such that the intensities detected are maximized at Talbot planes or at the midpoints between Talbot planes. The optical distance between the spatially periodic object and the detector is changed in order to detect image intensities at different Talbot planes (i.e., at the Talbot planes themselves as well as at intermediate distances between the planes). In another embodiment, the detector and the spatially periodic object are positioned along a common optical axis at relative angle xcex8 such that different rows of a detector array detect intensities at different Talbot planes. In yet another embodiment, the spatially periodic object is both a grating and a detector, and the Talbot images generated by the grating are reflected off a mirror back to the detector. Such a spatially periodic object may comprise, for example, a transparent substrate patterned with doped-intrinsic-doped regions. The optical distance between the mirror and the spatially periodic object may be changed, or the mirror and the spatially periodic object may be positioned along a common optical axis at relative angle xcex8 such that different detector rows detect intensities at different Talbot planes.
This novel class of transform spectrometers are simple, compact, low-cost, high precision, and versatile. These novel transform spectrometers are well suited for a variety of applications involving spectral imaging and sensing. Unlike typical transform spectrometers, embodiments of the present invention may be constructed without any moving parts, thus increasing reliability. The spectrometer also enjoys the multiplexing advantage of other types of transform spectrometers, i.e. the presence of all frequencies of light at the detector at all times provides an increased signal-to-noise ratio. As with other more common transform spectrometers, there is no free spectral range to limit the spectral sensing region. Only the detector response limits the range. Hence spectrometers of the present invention may be useful when a large wavelength range is needed in a compact device.